Progressing cavity pumps have been used in water wells for many years. More recently, such pumps have been found well suited for the pumping of viscous or thick fluids such as crude oil laden with sand. Progressing cavity pumps include a stator which is attached to a production tubing at the bottom of a well and a rotor which is attached to the bottom end of a pump drive string and is made of metallic material, usually high strength steel. The rotor is usually electro-plated with chrome to resist abrasion, but the corrosive and abrasive properties of the fluids produced in oil wells frequently cause increased wear and premature failure of the pump rotor. Since it is important for efficient operation of the pump that a high pressure differential be maintained across the pump, only small variations in the rotor's dimensions are tolerable. This means that excessively worn rotors must be replaced immediately. However, replacement of the rotor requires pulling the whole pump drive string from the well which is costly, especially in the deep oil well applications which are common for progressing cavity pumps. Consequently, pump rotors with increased wear resistance and, thus, a longer service life are desired to decrease well operating cost.
Various hardfacing methods have been used in the past to increase the wear resistance of metal surfaces. Hardfacings consisting of a thin layer of metal carbide applied by conventional plasma jet spraying techniques are the most commonly used due to the extreme hardness of the coating achieved. However, although this type of hardfacing works well when in friction contact with a metal surface, surfaces so coated have a roughness which makes them unacceptable for use in progressing cavity pump applications. The surface roughness of the metal carbide hardfacing is due to the grainy structure of the hardfacing structure which is caused by the individual sprayed-on metal carbide particles. This roughness results in excessive wear of the progressing cavity pump stator which is made of an elastomeric material, most often rubber. Polishing of the metal carbide hardfacing to overcome this problem is theoretically possible, but cannot be done economically due to the extreme hardness of the material. Thus, an economical hardfacing for progressing cavity pump rotors is desired which increases the surface life of the rotor without increasing stator wear. In particular, a hardfacing is desired which provides the surface hardness and wear characteristics of a metal carbide coating without having the same surface roughness.
The hardfacing of metal surfaces with tungsten carbide or tungsten carbide containing metal powders by plasma spraying or detonation gun is well known in the art and is disclosed in the following references.
Canadian Patent 746,458, McFarland et al; PA1 Canadian Patent 785,248, Rath; PA1 Canadian Patent 1,326,414, Jackson et al; PA1 U.S. Pat. No. 3,615,009, Prasse; PA1 European Patent Application 0,018,265, Bonnin; PA1 British Patent 1,434,365, Land et al; PA1 British Patent Application 2,083,079, Tenkula et al. PA1 plasma spraying a metal carbide material onto the rotor body to form a metal carbide hardfacing layer having a grainy surface with a multiplicity of peaks and intermediate depressions, the peaks being formed by metal carbide grains on the surface of the hardfacing layer, PA1 coating a top layer of metallic material onto the hardfacing layer to at least fill in the depressions intermediate the peaks, the metallic material being selected to have a lower hardness than the metal carbide; and PA1 polishing the top layer until the rotor is smooth and has dimensions within selected tolerances, and preferably until a majority of the peaks of the hardfacing layer are exposed to achieve a hardfacing surface of significantly reduced surface roughness. PA1 a ferrous metal body; PA1 a layer of a metal carbide material bonded to the body and having a grainy surface with a multiplicity of peaks and intermediate depressions, the peaks being formed by metal carbide grains at the surface of the first layer; and PA1 a top layer of metallic material bonded to the carbide layer, the thickness of the top layer being adjusted such that the depressions between the peaks of the first layer are completely filled while the majority of the peaks are exposed at the surface of the rotor, thereby providing the rotor with a metal carbide hardfacing of significantly reduced surface roughness.
Jackson et al (CA1,326,414), Bonnin (P0,018,265), Tenkula et al (GB2,083,079) and Rath (CA785,248), all disclose hardfacing layers made, from plasma sprayed metal powders containing tungsten carbide together with additional metals such as cobalt, chromium, chromium oxide, chromium carbide, nickel, nickel/chromium, or iron. These additional metals are added to provide the coating with improved corrosion resistance and/or bonding. McFarland et al (CA746,458) teach a process for the application of a protective nickel/chromium alloy fusion coating onto a metal base to provide the base with improved corrosion resistance. Land et al (GB1,434,365) discuss mechanical seals wherein one of the seal surfaces is a metal alloy carbide. A plasma sprayed boron carbide coating is applied to the other sealed surface to provide the mechanical seals with increased corrosion and wear resistance. Thus, although various hardfacings are disclosed, all these references are directed to methods and coatings for the achievement of improved corrosion and wear resistance of the coated metal surfaces. No guidance can be found therein towards a solution for the increased wear problems expected at metal/rubber interfaces with plasma sprayed hardfacings of the metal surface.
The invention now provides a multiple layer hardfacing for a progressing cavity pump rotor which overcomes the problem of excessive stator wear experienced in progressing cavity pumps having rotors with conventional metal carbide hardfacings.